That error means the AG failed one of the health detection timeouts (lease, session, or health check) summarized in this table from the docs:
Mechanics and guidelines of lease, cluster, and health check timeouts for Always On availability groups - Summary of Timeout Guidelines
Your first stop should be to review the SQL Server error log on each node in the AG, to see if there was something going on that might have caused the instance to stop responding to the cluster AG resource. For instance, you might have crash dumps, deadlocked schedulers, etc.
To dig into the cluster-related details, you'll need to call the PowerShell Get-ClusterLog cmdlet (see here) to get the cluster log file from each node in the AG. Then find the time associated with the failure error message you mentioned in the question. Review the cluster log files around that time period for errors (search for "ERR") or any messages of interest from the AG resource DLL itself (which will include "[RES]" or "[hadrag]").
I would expect you'll find messages that include "timed out" or "IsAlive" showing the timing of the failures. At this point, you should be able to figure out which server was not responding, and investigate that directly.
Figuring these things can be difficult. Microsoft support has released a tool that can be used to categorize some types of failovers, which might be useful to you:
Failover Detection Utility - Availability Group Failover Analysis Made Easy
